The following patent publications are believed to represent the current state of the art:
U.S. Pat. Nos. 5,694,468; 5,825,874 and 6,002,765; and
U.S. Patent Publication Nos. 2002/0009194; 2002/0176571; 2003/0013500; 2004/0204166 and 2004/0235533.